1. Field of the Invention
This invention relates to a weld control for spot welding a laminated workpiece including metal sheets and a viscoelastic material located between the metal. More particularly, the invention pertains to a method for detecting imminent and actual contact of the metal laminas with one another and a method for optimizing the length of the period that precedes the viscoelastic material being forced away from the weld area prior to welding the metal.
2. Description of the Prior Art
When electrical resistance spot welds are made in a workpiece that includes outer laminas of metal, such as low carbon steel or high strength steel, separated by an intermediate, non-metallic lamina such as a viscoelastic thermoplastic material, an unacceptable variation in weld quality can result. When the weld is made in the conventional way by passing electrical current between the welding electrode and through the workpiece at a current amplitude and during a period that is suitable for making a good spot weld in a workpiece that does not contain the separating lamina, the resulting weld can vary in quality from one that is only partially made to one that is substantially undersized in relation to acceptable standards. The quality of the weld is unacceptable because there is a partial or complete loss of metal-to-metal contact between the metal laminas during the period when electrical current is applied at welding amplitudes to the electrodes.
The viscoelastic lamina is substantially less conductive thermally and electrically than the metal and its presence between the metal laminas prevents the completion of a low resistance electrical circuit between the electrodes.
A spot weld is produced by a dynamic process wherein the metal is melted initially on an axis that is equally distant from the electrodes and then axially toward the electrodes and radially outward from the axis. The electrodes apply a clamping force to the workpiece which confines the melted metal. The process is stopped by terminating the welding current before the melt exceeds the electrode diameter. Otherwise, an unacceptable shower of sparks and hot metal will issue from the weld in a process caused expulsion. The magnitude of the weld current and resistivity of the workpiece determine the speed at which a weld nugget is produced. Metal expulsion that produces the shower of sparks sets one limit for the current; however, when weld current is too low, merely lengthening the period during which the power is supplied to the electrodes may not produce an acceptable weld nugget. The electrical resistivity of the workpiece is an important factor affecting nugget size. The temperature at which each element of the workpiece enters the plastic range is another important variable.
The electrical current shunt path through one or more completed welds adjacent to the weld currently being made draws electrical current from the location where the weld is being made. The indicated weld current supplied to the electrodes is greater than the current actually applied to the weld spot.
In making spot welds in laminated workpieces of the type described, weld quality depends on the rate at which the intermediate thermoplastic lamina is forced radially outward from the vicinity of the electrodes due to the application of pressure at the electrodes and whether there is metal-to-metal contact between the metal laminas when the welding cycle begins. In a conventional electrical resistant spot weld control, the length of the period during which laminated workpieces of this type are preheated to produce flow of the intermediate thermoplastic material away from the electrodes and the length of time during which current at welding magnitudes is applied to the electrodes are preselected, fixed values. In actual practice, the length of time required to assure that the intermediate thermoplastic material has moved from the vicinity of the electrodes prior to welding varies substantially depending on the condition of the electrodes, the material of the workpiece, the surface condition of the workpiece and other parameters that are difficult to quantify. It is preferable that some means be made available to account for the widely varying length of time needed to assure that metal-to-metal contact between the metal laminas exists before welding current is applied.